Method for producing polarizer

ABSTRACT

A method for producing a polarizer comprises the steps of: (A) stretching a polyvinyl alcohol-based resin layer to obtain a stretched layer; (B) immersing the stretched layer in a dyeing liquid containing iodine to obtain a dyed layer in which absorbance thereof determined from a tristimulus value Y is from 0.4 to 1.0 (transmittance T=40% to 10%); and (C) removing a part of iodine adsorbed in the dyed layer so that the absorbance of the dyed layer decreases by 0.03 to 0.7, provided that the absorbance of the dyed layer is controlled so that it does not become less than 0.3.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a polarizerincluding a polyvinyl alcohol-based resin layer containing iodine.

2. Description of Related Art

There has been known a production method in which a polyvinyl alcoholfilm is dyed with a dyeing liquid containing iodine and then stretchedto obtain a polarizer (for example, JP-A-2003-270440). In theabove-mentioned production method, the thus dyed and stretched film issubjected to an iodine ion impregnation treatment by immersing in anaqueous potassium iodide solution, and then subjected to an alcoholliquid immersion treatment by immersing in an alcohol liquid.

The polarizer obtained by this production method exhibits lessyellowness and is less likely to cause a change in color hue even undera heating environment. The reason why the polarizer exhibits lessyellowness is that absorbance thereof is nearly constant in the entirewavelength range of visible light.

However, the above-mentioned polarizer has a problem such as a lowpolarization degree.

SUMMARY OF THE INVENTION

There has been known, as a method of obtaining a polarizer, a method inwhich a polyvinyl alcohol film is dyed with a dyeing liquid containingiodine, stretched, subjected to an iodine ion impregnation treatment byimmersing in an aqueous potassium iodide solution, and then subjected toan alcohol liquid immersion treatment by immersing in an alcohol liquid.However, the polarizer obtained by this production method has a problemsuch as a low polarization degree.

The present invention provides a production method capable of obtaininga polarizer including a polyvinyl alcohol-based resin layer containingiodine, which has a high polarization degree.

The summary of the present invention is as follows:

In a first preferred aspect of the present invention, there is provideda method for producing a polarizer including a polyvinyl alcohol-basedresin layer having a layer thickness of 0.6 μm to 5 μm, and containingiodine. The production method of the present invention includes thesteps of:

-   -   (A) stretching a polyvinyl alcohol-based resin layer to obtain a        stretched layer;    -   (B) immersing the stretched layer in a dyeing liquid containing        iodine to obtain a dyed layer in which absorbance thereof        determined from a tristimulus value Y is from 0.4 to 1.0        (transmittance T=40% to 10%); and    -   (C) removing a part of iodine adsorbed in the dyed layer so that        the absorbance of the dyed layer decreases by 0.03 to 0.7,        provided that the absorbance of the dyed layer is controlled so        that it does not become less than 0.3.

In a second preferred aspect of the method according to the presentinvention, the polyvinyl alcohol-based resin layer is formed on asupport and the polyvinyl alcohol-based resin layer is stretchedtogether with the support in the step A.

In a third preferred aspect of the method according to the presentinvention, the stretching method is dry stretching in the step A.

In a fourth preferred aspect of the method according to the presentinvention, the temperature at the time of stretching (stretchingtemperature) is from 130° C. to 170° C. in the step A.

In a fifth preferred aspect of the method according to the presentinvention, the absorbance is from 0.4 to 1.0 (T=40% to 10%) when thedyed layer has a layer thickness of 0.6 μm to 5 μm in the step B.

In a sixth preferred aspect of the method according to the presentinvention, the dyeing liquid is an aqueous solution containing iodineand alkali iodide or ammonium iodide in the step B.

In a seventh preferred aspect of the method according to the presentinvention, a part of iodine adsorbed in the dyed layer is removed byimmersing the dyed layer in a decolorization liquid containing alkaliiodide or ammonium iodide in the step C.

In an eighth preferred aspect of the method according to the presentinvention, the polarizer has an absorbance of 0.3 to 0.4 (T=50% to 40%)at the time point of completion of the step C.

In a ninth preferred aspect, the method according to the presentinvention further includes a step of immersing the dyed layer or thedyed layer partially decolorized in a crosslinking liquid containingboric acid and alkali iodide or ammonium iodide.

In a tenth preferred aspect, the method according to the presentinvention further includes a step of drying the polarizer.

The present inventors have intensively studied so as to achieve theabove-mentioned object and found that a polarizer having a highpolarization degree is obtained by sequentially subjecting to thefollowing three steps A to C. The respective steps will be describedwith reference to FIGS. 1 and 2. FIGS. 1 and 2 show the case where apolyvinyl alcohol-based resin layer is formed on a support and thepolyvinyl alcohol-based resin layer is stretched together with thesupport.

FIG. 1( a) is a schematic sectional view of a polyvinyl alcohol-basedresin layer 10 before stretching formed on a support 30. The polyvinylalcohol-based resin layer 10 is composed of an amorphous portion 11 anda crystallized portion 12. The crystallized portion 12 exists at randomin the amorphous portion 11. Arrow 13 shows a stretch direction in thesubsequent step.

[Step A] Stretching Before Dyeing

First, the polyvinyl alcohol-based resin layer 10 is stretched togetherwith a support 30. The polyvinyl alcohol-based resin layer 10 isreferred to as a stretched layer 14 after stretching. FIG. 1( b) is aschematic sectional view of the stretched layer 14. A first point in theproduction method of the present invention is that the polyvinylalcohol-based resin layer 10 is stretched before dyeing. Arrow 15 showsa stretch direction. A polymer chain (not shown) in the stretched layer14 is crystallized by stretching to form a crystallized portion 17having higher orientation property in an amorphous portion 16.

[Step B] Excessive Dyeing

Then, the stretched layer 14 is dyed. Dyeing is an adsorption treatmentof iodine. The stretched layer 14 is referred to as a dyed layer 18after dyeing. FIG. 2( c) is a schematic sectional view of the dyed layer18. A second point in the production method of the present invention isthat the stretched layer 14 is immersed in a dyeing liquid containingiodine and excessively dyed. Excessive dyeing means that dyeing iscarried out so that an absorbance A_(B) determined from a tristimulusvalue Y becomes 0.4 or more. A subscript B of the absorbance A_(B)represents the step B.

A common polarizer has an absorbance A, which is determined from thetristimulus value Y, of about 0.37. For example, a polarizer with atransmittance T of 43% has an absorbance A of 0.367. Therefore, it issupposed that dyeing which enables an absorbance A_(B) of 0.4 or more,like dyeing in the present invention, is excessive dyeing.

The crystallized portion 17 of the polymer chain is not easily dyed ascompared with the amorphous portion 16. However, it is also possible tosufficiently adsorb iodine to the crystallized portion 17 by excessivelydyeing the stretched layer 14. The adsorbed iodine forms a polyiodineion complex 19 of I³⁻ or I⁵⁻ or the like in the dyed layer 18. Thepolyiodine ion complex 19 exhibits absorption dichroism in a visiblelight range (wavelength of 380 nm to 780 nm).

[Step C] Decolorization (Partial Removal of Iodine)

Next, a part of iodine adsorbed to the dyed layer 18 is removed. Thisoperation is referred to as decolorization. The dyed layer 18 isreferred to as a polarizer 20 after decolorization. The iodine isadsorbed to the dyed layer 18 in the form of the polyiodine ion complex19. FIG. 2( d) is a schematic sectional view of the polarizer 20. Athird point in the production method of the present invention is that apart of the polyiodine ion complex 19 is removed from the excessivelydyed layer 18. In order to remove the polyiodine ion complex 19 from thedyed layer 18, for example, the dyed layer 18 is immersed in an aqueouspotassium iodide solution (decolorization liquid). At this time, thepolyiodine ion complex 19 is removed so that the absorbance A_(C)decreases by 0.03 to 0.7, provided that the absorbance of the dyed layeris controlled so that it does not become less than 0.3. A subscript C ofthe absorbance A_(C) represents the step C.

When the polyiodine ion complex 19 is removed, the polyiodine ioncomplex 19 adsorbed to the amorphous portion 16 is preferentiallyremoved. As a result, the polyiodine ion complex 19 adsorbed to thecrystallized portion 17 remains in a relatively large amount.

The polyiodine ion complex 19 adsorbed to the amorphous portion 16slightly contributes to absorption dichroism. On the other hand, thepolyiodine ion complex 19 adsorbed to the crystallized portion 17largely contributes to absorption dichroism. However, the polyiodine ioncomplex 19 adsorbed to the amorphous portion 16 and the polyiodine ioncomplex 19 adsorbed to the crystallized portion 17 increase theabsorbance, in the same way. According to the production method of thepresent invention, it is possible to preferentially remove thepolyiodine ion complex 19 adsorbed to the amorphous portion 16, whichincreases the absorbance regardless of small contribution to absorptiondichroism. Therefore, the amount of the polyiodine ion complex 19adsorbed to the crystallized portion 17, which largely contributes toabsorption dichroism, relatively increases. Large contribution toabsorption dichroism means a high polarization degree. Thus, accordingto the production method of the present invention, it is possible toobtain a polarizer 20 having a high polarization degree regardless oflow absorbance.

ADVANTAGE OF THE INVENTION

According to the production method of the present invention, the amountof the polyiodine ion complex 19 adsorbed to the crystallized portion17, which largely contributes to absorption dichroism, increases, thusobtaining a polarizer 20 having a high polarization degree regardless oflow absorbance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic view showing a preliminary step of productionstep of the present invention, and FIG. 1( b) is a schematic viewshowing a production step A of the present invention.

FIG. 2( c) is a schematic view showing a production step B of thepresent invention, and FIG. 2( d) is a schematic view showing aproduction step C of the present invention.

FIG. 3 is a schematic view showing a production step of the presentinvention.

FIG. 4 is a graph of absorbance versus polarization degree of apolarizer.

FIG. 5 is a graph of absorbance versus polarization degree of a dyedlayer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Production Method of thePresent Invention]

FIG. 3 is a schematic view showing a production step of the presentinvention. The production method of the present invention is a methodfor producing a polarizer 20 including a polyvinyl alcohol-based resinlayer containing iodine.

A film-shaped untreated polyvinyl alcohol-based resin layer 10 formed ona support 30 is sequentially pulled out from a feed portion 21, togetherwith the support 30, for a treatment.

In the step A, the polyvinyl alcohol-based resin layer 10 is stretchedwhile passing through stretch rolls 22, together with the support 30, toform a stretched layer 14.

In the step B, the stretched layer 14 is immersed in a dyeing liquid 23containing iodine to form a dyed layer 18. The dyed layer 18 has anabsorbance A_(B), which is determined from the tristimulus value Y, of0.4 to 1.0 (T=40% to 10%).

In the step C, the dyed layer 18 is immersed in a decolorization liquid24 (aqueous potassium iodide solution) thereby removing a part of iodineto form a polarizer 20. At this time, the absorbance A_(C) of thepolarizer 20 decreases by 0.03 to 0.7 as compared with the absorbanceA_(B) of the dyed layer 18 immediately before the step C, provided thatthe absorbance of the dyed layer is controlled so that it does notbecome less than 0.3.

The thus completed polarizer 20 is wound around a take-up portion 25.

The production method of the present invention may include the othersteps as long as it includes the above-mentioned steps A, B and C inthis order. Examples of the other steps include a step in which the dyedlayer 18 is immersed in a crosslinking liquid (aqueous solutioncontaining boric acid and, optionally, potassium iodide) between thestep B and the step C thereby crosslinking the polyvinyl alcohol-basedresin layer, and the step of drying the polarizer 20 obtained in thestep C.

[Step A]

The step A to be used in the present invention is a step of stretchingthe polyvinyl alcohol-based resin layer 10 to obtain the stretched layer14.

The polyvinyl alcohol-based resin layer 10 to be used in the presentinvention is obtained by forming a polyvinyl alcohol-based resin intothe form of a layer. The polyvinyl alcohol-based resin layer 10 ispreferably formed on the support 30.

The polyvinyl alcohol-based resin is typically obtained by saponifying apolyvinyl acetate-based resin. The polyvinyl alcohol-based resin to beused in the present invention is, for example, polyvinyl alcohol or anethylene-vinyl alcohol copolymer. The saponification degree of thepolyvinyl alcohol-based resin to be used in the present invention ispreferably from 85 mol % to 100 mol %, more preferably from 95 mol % to100 mol %, and still more preferably from 98 mol % to 100 mol %, sincewater resistance is enhanced and it becomes possible to stretch at ahigh stretch ratio. The polymerization degree of the polyvinylalcohol-based resin to be used in the present invention is preferablyfrom 1,000 to 10,000 since it is possible to increase the polarizationdegree by increasing the amount of the polyiodine ion complex adsorbedto the crystallized portion 17.

It is possible to use, as a method of stretching the polyvinylalcohol-based resin layer 10, any known stretching methods such as aroll stretching method and a tenter stretching method. The stretch ratioof the polyvinyl alcohol-based resin layer 10 is usually from 3 to 7times larger than the original length.

Stretching of the polyvinyl alcohol-based resin layer 10 is preferablydry stretching. Dry stretching is stretching in air. Dry stretching ispreferable than wet stretching since it can increase the crystallizationdegree. In this case, the stretching temperature is preferably from 80°C. to 170° C., and more preferably from 130° C. to 170° C. It ispossible to accelerate the crystallization of a polymer chain in thepolyvinyl alcohol-based resin layer 10 by setting the stretchingtemperature to 130° C. or higher. As a result, it is possible toincrease the amount of the polyiodine ion complex 19 adsorbed to thecrystallized portion 17, and thus the polarization degree can beincreased. It is also possible to prevent the crystallization of apolymer chain from excessively promoting and to shorten the dyeing timein the step B by setting the stretching temperature to 170° C. or lower.The polyvinyl alcohol-based resin layer 10 is preferably stretched sothat the crystallization degree after stretching becomes 20% to 50%, andmore preferably 32% to 50%. When the crystallization degree afterstretching is from 20% to 50%, the amount of the polyiodine ion complex19 adsorbed to the crystallized portion 17 increases, and thus thepolarization degree can be increased.

The polyvinyl alcohol-based resin layer 10 may contain other additives,in addition to iodine. Examples of the other additives include,surfactants, antioxidants, crosslinking agents and the like.

The polyvinyl alcohol-based resin layer 10 before stretching has a layerthickness t₀ of usually 2 μm to 30 μm, and preferably 3 μm to 15 μm.Since the polyvinyl alcohol-based resin layer 10 has a thin layerthickness before stretching, in the case where it is difficult tostretch alone, the polyvinyl alcohol-based resin layer 10 is formed onthe support 30 and the polyvinyl alcohol-based resin layer 10 isstretched, together with the support 30.

The stretched layer 14 has a layer thickness t₁ of usually 0.4 μm to 7μm, and preferably 0.6 μm to 5 μm. When the stretched layer 14 has alayer thickness t₁ of 5 μm or less, it is possible to achieve theobjective absorbance by dyeing within a short time.

[Step B]

In the step B to be used in the present invention, the stretched layer14 obtained in the step A is immersed in the dyeing liquid 23 containingiodine to obtain the dyed layer 18. The dyed layer 18 has an absorbanceA_(B) of preferably 0.4 to 1.0 (T=40% to 10%). The dyed layer 18 has anabsorbance A_(B) of more preferably 0.5 to 1.0 (T=31.6% to 10%). Whenthe dyed layer 18 has an absorbance A_(B) of less than 0.4 (case where Tis more than 40%), the polyiodine ion complex 19 may not be sometimesadsorbed sufficiently to the crystallized portion 17 of the polymerchain.

In the present invention, the absorbance A is calculated by the equation(1):

A=log₁₀(1/T)  (1)

wherein the transmittance T is a value of the tristimulus value Y of theXYZ calorimetric system based on a two-degree view field in accordancewith the JIS Z 8701 (1995). In the present specification, a value of thetransmittance T is represented by a percentage assuming that it is 100%when T=1.

The absorbance A_(B), which is determined from the tristimulus value Y,of the dyed layer 18 may vary if it falls within a defined range (0.4 to1.0) immediately after the step B.

The dyeing liquid 23 to be used in the present invention is usually anaqueous solution containing iodine and alkali iodide or ammonium iodide.In the dyeing liquid 23, alkali iodide or ammonium iodide is used so asto enhance solubility of iodine in water. The content of iodide of thedyeing liquid 23 is preferably from 1.1 parts by weight to 5 parts byweight based on 100 parts by weight of water. When potassium iodide isused as alkali iodide, the content of potassium iodide of the dyeingliquid 23 is preferably from 3 parts by weight to 30 parts by weightbased on 100 parts by weight of water.

The temperature and immersion time of the dyeing liquid 23 areappropriately set so as to satisfy properties defined in the presentinvention depending on the concentration of the dyeing liquid 23 and thelayer thickness of the stretched layer 14. The temperature of the dyeingliquid 23 is preferably from 20° C. to 40° C. The time of immersion inthe dyeing liquid 23 is preferably from 60 seconds to 1,200 seconds.

[Step C]

In the step C to be used in the present invention, a part of thepolyiodine ion complex 19 is removed from the dyed layer 18 obtained inthe step B to obtain the polarizer 20. The absorbance A_(C) of thepolarizer 20 is controlled to the value which is 0.03 to 0.7 less thanthe absorbance A_(B) of the dyed layer 18 by removing the polyiodine ioncomplex 19. Provided that the absorbance A_(B) of the dyed layer iscontrolled so that it does not become less than 0.3.

The polarizer 20 has an absorbance A_(C), which is determined from thetristimulus value Y, of preferably 0.3 to 0.4 (T=50% to 40%). In orderto obtain the absorbance A_(T) within the above-mentioned range, thewidth ΔA (=A_(B)−A_(C)) of a decrease in absorbance in the step C ispreferably from 0.03 to 0.7. The width ΔA of a decrease in absorbance inthe step C is more preferably from 0.05 to 0.65. When the width ΔA of adecrease in absorbance is less than 0.03, a polarizer 20 having a highpolarization degree may not be sometimes obtained.

When a part of the polyiodine ion complex 19 is removed from the dyedlayer 18, for example, an aqueous solution of alkali iodide or ammoniumiodide is used. The aqueous solution of alkali iodide or ammonium iodideused for this purpose is referred to as a decolorization liquid 24. Thetreatment of removing a part of the polyiodine ion complex 19 from thedyed layer 18 is referred to as decolorization. Decolorization may becarried out by immersing the dyed layer 18 in the decolorization liquid24, or the decolorization liquid may be applied or sprayed on a surfaceof the dyed layer 18.

In the decolorization liquid 24, the polyiodine ion complex 19 is apt toelute from the dyed layer 18 by an action of iodine ions. Iodine ionsare obtained from alkali iodides such as potassium iodide, sodiumiodide, lithium iodide, magnesium iodide and calcium iodide.Alternatively, iodine ions are obtained from ammonium iodide. It ispreferred that the concentration of iodine ions in the decolorizationliquid 24 is sufficiently less than that of the dyeing liquid 23. Whenpotassium iodide is used, the content of potassium iodide in thedecolorization liquid 24 is preferably from 1 part by weight to 20 partsby weight based on 100 parts by weight of water.

The temperature and immersion time of the decolorization liquid 24 areappropriately set according to the layer thickness of the dyed layer 18.The temperature of the decolorization liquid 24 is preferably from 45°C. to 75° C. The immersion time in the decolorization liquid 24 ispreferably from 20 seconds to 600 seconds.

[Polarizer Obtained by the Production Method of the Present Invention]

The polarizer 20 obtained by the production method of the presentinvention includes a polyvinyl alcohol-based resin layer containingiodine. The above-mentioned polyvinyl alcohol-based resin layer isstretched and dyed, and thus polymer chains are oriented in a givendirection. Iodine forms the polyiodine ion complex 19 such as I³⁻ or I⁵⁻in the polyvinyl alcohol-based resin layer and exhibits absorptiondichroism within a visible light range (wavelength 380 nm to 780 nm).

The film thickness t₃ of the polarizer 20 is usually the same as thelayer thickness t₁ of the stretched layer 14 and is usually from 0.4 μmto 7 μm, and preferably from 0.6 μm to 5 μm.

According to the production method of the present invention, it ispossible to adjust the polarization degree of the polarizer 20 having anabsorbance A_(C) of about 0.37 (T=43%) and a film thickness t₃ of 5 μmor less to 99.9% or more.

EXAMPLES Example 1

(1) An aqueous 7% by weight solution of polyvinyl alcohol was applied ona surface of a support made of a norbornene-based resin film having afilm thickness of 150 μm (manufactured by JSR Corporation; product name:ARTON) to form a polyvinyl alcohol film. The polymerization degree ofpolyvinyl alcohol was 4,200, and the saponification degree thereof was99% or more.

(2) The polyvinyl alcohol film and the support were dried at 80° C. for8 minutes to form a polyvinyl alcohol layer having a layer thickness of7 μm on the support to obtain a laminate of the polyvinyl alcohol layerand the support.

(3) Using a biaxial stretching machine manufactured by IwamotoSeisakusho Co., Ltd., the laminate of the polyvinyl alcohol layer andthe support was subjected to dry uniaxial stretching. The stretchingtemperature was 150° C. The stretch ratio was adjusted to the valuewhich is 4.8 times larger than the original length. As a result ofstretching, a laminate of the stretched layer and the support wasobtained. The support is also stretched at the same ratio as that of thestretched layer.

(4) The laminate of the stretched layer and the support was immersed ina dyeing liquid of an aqueous solution containing iodine and potassiumiodide thereby adsorbing and orienting a polyiodine ion complex to thestretched layer to obtain a laminate of the dyed layer and the support.The immersion time in the dyeing liquid was 600 seconds. The liquidtemperature of the dyeing liquid was 25° C. The composition of thedyeing liquid was as follows: iodine: potassium iodide:water=1.1:7.8:100 in term of a weight ratio. Immediately after dyeing,the absorbance was 0.602.

(5) The laminate of the dyed layer and the support was immersed in adecolorization liquid containing potassium iodide and a part of thepolyiodine ion complex of the dyed layer was removed. The composition ofthe decolorization liquid was as follows: water:potassium iodide=100:5.3in terms of a weight ratio. The liquid temperature of the decolorizationliquid was 60° C. The immersion time in the decolorization liquid wasadjusted and five kinds of samples of the obtained polarizer having anabsorbance of 0.357 to 0.377 were made.

(6) A laminate of the dyed layer partially decolorized and the supportwas immersed in a crosslinking liquid containing boric acid andpotassium iodide. The composition of the crosslinking liquid was asfollows: water:boric acid:potassium iodide=100:11.8:5.9 in terms of aweight ratio. The immersion time in the crosslinking liquid was 60seconds. The liquid temperature of the crosslinking liquid was 60° C.

(7) A laminate of the dyed layer subjected to a crosslinking treatmentand the support was dried at 60° C. for 120 seconds.

The laminate of a polarizer (film thickness of 2.9 μm) and the supportwas formed by the above-mentioned procedure. A graph of the absorbance(A_(C)) versus polarization degree of the polarizer is shown in FIG. 4.A graph of the absorbance (A_(B)) versus polarization degree of the dyedlayer is shown in FIG. 5.

Example 2

A laminate composed of a polarizer (thickness: 2.9 μm) and a support wasformed in the same manner as in Example 1 except for the followingpoints:

-   -   (1) The immersion time in the dyeing liquid was adjusted to set        an absorbance at 0.921 immediately after dyeing.    -   (2) The immersion time in the decolorization liquid was adjusted        to prepare five kinds of samples of the obtained polarizer        having an absorbance of 0.359 to 0.377.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer. And FIG. 5 is a graph of absorbance (A_(B)) versuspolarization degree of a dyed layer.

Example 3

A laminate composed of a polarizer (thickness: 2.9 μm) and a support wasformed in the same manner as in Example 1 except for the followingpoints:

-   -   (1) The immersion time in the dyeing liquid was adjusted to set        an absorbance at 0.420 immediately after dyeing.    -   (2) The immersion time in the decolorization liquid was adjusted        to prepare four kinds of samples of the obtained polarizer        having an absorbance of 0.362 to 0.377.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer. And FIG. 5 is a graph of absorbance (A_(B)) versuspolarization degree of a dyed layer.

Example 4

A laminate composed of a polarizer (thickness: 2.9 μm) and a support wasformed in the same manner as in Example 1 except for the followingpoints:

-   -   (1) The immersion time in the dyeing liquid was adjusted to set        an absorbance at 0.959 immediately after dyeing.    -   (2) The immersion time in the decolorization liquid was adjusted        to prepare four kinds of samples of the obtained polarizer        having an absorbance of 0.357 to 0.376.    -   (3) The stretching temperature was 100° C. and the stretch ratio        was 4.5 times larger than the original length.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer

Example 5

A laminate composed of a polarizer (thickness: 3.5 μm) and a support wasformed in the same manner as in Example 1 except for the followingpoints:

-   -   (1) An aqueous 5% by weight solution of polyvinyl alcohol was        applied onto a surface of the support.    -   (2) The stretching temperature of the laminate was 140° C. and        the stretch ratio was 4.0 larger than the original length.    -   (3) The composition of the dyeing liquid was as follows: iodine:        potassium iodine: water=1:7:92 in terms of a weight ratio. The        immersion time in the dyeing liquid was 300 seconds.    -   (4) The absorbance immediately after dyeing was 0.613.    -   (5) The content of potassium iodide of the decolorization liquid        was as follows: water: potassium iodine=95:5 in terms of a        weight ratio. The immersion time in the decolorization liquid        was 30 seconds and the obtained polarizer had an absorbance of        0.380.    -   (6) The composition of the crosslinking liquid was as follows:        water: boric acid: potassium iodine=85:1:0:5 in terms of a        weight ratio.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer.

Example 6

A laminate composed of a polarizer (thickness: 3.5 μm) and a support wasformed in the same manner as in Example 5 except for the followingpoints:

-   -   (1) The immersion time in the dyeing liquid was 600 seconds. The        absorbance immediately after dyeing was 0.417.    -   (2) The immersion time in the decolorization liquid was 2        seconds. The obtained polarizer had an absorbance of 0.380.

FIG. 4 is a graph of a graph of absorbance (A_(C)) versus polarizationdegree of a polarizer.

Example 7

A laminate composed of a polarizer (thickness: 3.5 μm) and a support wasformed in the same manner as in Example 5 except for the followingpoints:

-   -   (1) An amorphous polyethylene terephthalate film with a        thickness of 200 μm (manufactured by Mitsubishi Plastics, Inc.,        product name: Novaclear SG007) was used as a support.    -   (2) The stretching temperature was 110° C.    -   (3) The composition of the dyeing liquid was as flows: iodine:        potassium iodine: water=0.2:1.4:98.4 in terms of a weight ratio.    -   (4) The immersion time in the dyeing liquid was 600 seconds. The        absorbance immediately after dyeing was 0.577.    -   (5) The immersion time in the decolorization liquid was 8        seconds and the obtained polarizer had an absorbance of 0.380.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer.

Comparative Example 1

A laminate composed of a polarizer (thickness: 2.9 μm) and a support wasformed in the same manner as in Example 1 except for the followingpoints:

-   -   (1) The immersion time in the dyeing liquid was adjusted to        prepare four kinds of samples of the obtained polarizer having        an absorbance of 0.367 to 0.387.    -   (2) No decolorization process was performed.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer. And FIG. 5 is a graph of absorbance (A_(B)) versuspolarization degree of a dyed layer.

Comparative Example 2

A laminate composed of a polarizer (thickness: 2.9 μm) and a support wasformed in the same manner as in Example 1 except for the followingpoints:

-   -   (1) The immersion time in the dyeing liquid was adjusted to        prepare four kinds of samples of the obtained polarizer having        an absorbance of 0.367 to 0.384.    -   (2) No decolorization process was performed.    -   (3) The stretching temperature was 100° C. and the stretch ratio        was 4.5 times larger than the original length.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer.

Comparative Example 3

A laminate composed of a polarizer (thickness: 3.5 μm) and a support wasformed in the same manner as in Example 5 except for the followingpoints:

(1) The composition of the dyeing liquid was as follows: iodine:potassium iodine: water=0.5:3.5:96.0.

(2) The immersion time in the dyeing liquid was 25 seconds. Theabsorbance immediately after dyeing was 0.395.

(3) No decolorization process was performed.

FIG. 4 is a graph of absorbance (A_(C)) versus polarization degree of apolarizer.

[Evaluation]

FIG. 4 illustrates a graph of absorbance (A_(C)) versus polarizer degreeof a polarizer.

-   -   (1) Comparing Examples 1 to 3, Example 2, Example 1, and Example        3 are arranged in descending order of reductions in polarization        degree caused by decolorization. This is the descending order of        absorption reduction caused by decolorization. The reduction of        absorption in Example 2 was 0.544 to 0.562. And the reduction of        absorption in Example 1 was 0.225 to 0.245. The reduction of        absorption in Example 3 was 0.043 to 0.058. It is, therefore,        presumed that the difference in polarization degree among        Examples 1 to 3 is caused by reductions in absorption caused by        decolorization.    -   (2) The polarization degree in Example 4 is higher than that of        Example 3. The stretch conditions of Example 4 are 150° C. and        4.5 times larger than the original length. The stretch        conditions in Example 3 are 150° C. and 4.8 times larger than        the original length. The stretch conditions are disadvantageous        in Example 4. On the other hand, the reduction of absorbance in        Example 4 was 0.583 to 0.602. The reduction of absorbance in        Example 3 was 0.043 to 0.058. This means that Example 4 is        advantageous regarding reductions in absorbance. Since        advantageous effects of the reduction in absorbance in Example 4        exceeded disadvantageous effects of the stretch conditions, it        is presumed that the polarization degree of Example 4 is higher        than that of Example 3.    -   (3) Stretch conditions in Example 5 are similar to the stretch        conditions in Example 6. However, while a reduction in        absorbance caused by decolorization is big (0.243) in Example 5,        a reduction in absorbance is small (0.037) in Example 6. As a        result, it is presumed that there is a difference between        Example 5 and Example 6 in polarization degree because of the        difference of reduction in absorbance.    -   (4) The stretching temperature (110° C.) in Example 7 is lower        than the stretching temperature (140° C.) in Examples 5 and 6.        Therefore, it is presumed to have a low polarization degree in        Example 7.    -   (5) It is presumed that the reason why the polarization degree        in Comparative Examples 1, 2, and 3 is low is that the reduction        in absorbance caused by decolarization has not been achieved.

FIG. 5 illustrates a graph of the absorbance (A_(r),) versuspolarization degree of a dyed layer. The stretch conditions in Examplesand Comparative Example are both 150° C. and 4.8 times larger than theoriginal length in this graph. Therefore, this graph simply showseffects of the reduction in absorbance caused by decolorization. Thisgraph plots absorbance when the absorbance (A_(C)) of the polarizationdegree was set at 0.367. Thus, the higher absorbance of the dyed layeris, there is more reduction in absorbance caused by decolorization.Example 2, Example 1, and Example 3 are arranged in descending order ofthe more reduction in absorbance caused by decolorization. Nodecolorizing was performed in Comparative Example 1. As can be seen fromthe graph, the more the reduction in absorbance occurs, the higher thepolarization degree is.

[Measuring Method] [Absorbance]

An absorbance A was calculated from the following equation (1) bymeasuring a transmittance T of a sample using a spectrophotometer withintegrating sphere (manufactured by MURAKAMI COLOR RESEARCH LABORATORYCO., LTD., product name: Dot-41):

A=log₁₀ (1/T)  (1)

wherein the transmittance T herein means a value of tristimulus value Yof the XYZ colorimetric systems based on a two-degree view field inaccordance with the JIS Z 8701 (1995).

Polarization Degree

A polarization degree was calculated from the following equation (2) bymeasuring a parallel transmittance H₀, and an orthogonal transmittanceH₉₀ of a sample using a spectrophotometer with integrating sphere(manufactured by MURAKAMI COLOR RESEARCH LABORATORY CO., LTD., productname: Dot-41):

Polarization degree (%)={(H ₀ −H ₉₀)/(H ₀ +H ₉₀)}^(1/2)×100  (2)

Parallel transmittance means a transmittance measured when the twopolarizers prepared in the same conditions are laminated so that thetransmission axes may be parallel to each other. Orthogonaltransmittance means a transmittance measured when the two polarizersprepared in the same manner are laminated such that transmittance axesthereof may be at right angles to each other. The parallel transmittanceand the orthogonal transmittance are respectively a value Y of thetristimulus value of the XYZ colorimetric systems based on a two-degreeview field in accordance with the JIS Z 8701 (1995).

INDUSTRIAL APPLICABILITY

The polarizer of the present invention is preferably used for liquidcrystal display devices such as liquid crystal television units,computer displays, car navigation systems, mobile phones, and gamedevices or the like.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   10: polyvinyl alcohol-based resin layer; 11: amorphous portion;        12: crystallized portion; 13: arrow indicating a stretch        direction; 14: stretched layer; 15: arrow indicating a stretch        direction; 16: amorphous portion; 17: crystallized portion; 18:        dyed layer; 19: polyiodine ion complex; 20: polarizer; 21: feed        portion; 22: stretch roll; 23: dyeing liquid; 24: decolorization        liquid; 25: take-up portion; 30: support

What is claimed is:
 1. A method for producing a polarizer including apolyvinyl alcohol-based resin layer having a layer thickness of 0.6 μmto 5 μm, and containing iodine, the method comprising the steps of: (A)stretching a polyvinyl alcohol-based resin layer to obtain a stretchedlayer; (B) immersing the stretched layer in a dyeing liquid containingiodine to obtain a dyed layer in which absorbance thereof determinedfrom a tristimulus value Y is from 0.4 to 1.0 (transmittance T=40% to10%); and (C) removing a part of iodine adsorbed in the dyed layer sothat the absorbance of the dyed layer decreases by 0.03 to 0.7, providedthat the absorbance of the dyed layer is controlled so that it does notbecome less than 0.3.
 2. The method according to claim 1, wherein thepolyvinyl alcohol-based resin layer is formed on a support and thepolyvinyl alcohol-based resin layer is stretched together with thesupport in the step A.
 3. The method according to claim 1 or claim 2,wherein the stretching method is dry stretching in the step A.
 4. Themethod according to claim 1 or claim 2, wherein the temperature at thetime of stretching is from 130° C. to 170° C. in the step A.
 5. Themethod according to claim 1 or claim 2, wherein the absorbance is from0.4 to 1.0 (T=40% to 10%) when the dyed layer has a layer thickness of0.6 μm to 5 μm in the step B.
 6. The method according to claim 1 orclaim 2, wherein the dyeing liquid is an aqueous solution containingiodine and alkali iodide or ammonium iodide in the step B.
 7. The methodaccording to claim 1 or claim 2, wherein a part of iodine adsorbed inthe dyed layer is removed by immersing the dyed layer in adecolorization liquid containing alkali iodide or ammonium iodide in thestep C.
 8. The method according to claim 1 or claim 2, wherein thepolarizer has an absorbance of 0.3 to 0.4 (T=50% to 40%) at the timepoint of completion of the step C.
 9. The method according to claim 1 orclaim 2, further comprising a step of immersing the dyed layer or thedyed layer partially decolorized in a crosslinking liquid containingboric acid and alkali iodide or ammonium iodide.
 10. The methodaccording to claim 1 or claim 2, still further comprising a step ofdrying the polarizer.